Introduction & Prior Art on Separable Membrane Technology
Electronic nebulisers that use ultrasonic vibration to generate liquid droplets are well known in the art and have found use in a wide range of fields including medical drug delivery and the treatment of air (for example fragrance delivery and humidification). A subset of such devices in widespread use (commonly referred to as ‘pond misters’) use a vibrating surface covered by liquid to cause droplets to be generated through the break-up of standing waves on the liquid free surface (U.S. Pat. No. 3,812,854 being an example). This break-up leads to droplets with a wide range of sizes being produced and shaping of the liquid container above the level of the liquid is used to limit the size range of droplets that escape and are delivered. With a wide range of droplets being contained and returned to the bulk liquid, such devices have low efficiency resulting in high power consumption. The efficiency of such devices can be improved by constraining the free surface of the liquid with a perforate membrane (U.S. Pat. No. 4,533,082 for example). This membrane may have just a single nozzle (for dispensing or printing applications for example in which individual drops may be dispensed on demand) or may have many thousand nozzles (for nebuliser applications for example). Relatively monodispersed droplets are produced when such perforate membranes are used in which the droplet diameter is related to the size of the openings, or nozzles, in the perforate membrane. Such devices still suffer multiple disadvantages: In particular, the vibrating surface needs to be mounted close to the membrane, but not touching, for effective droplet generation and not all liquid in the container can be delivered (as the liquid is required to transmit the pressure waves to the perforate membrane). A preferred embodiment of such devices is therefore one in which the perforate membrane itself is vibrated by the driver element (commonly called the actuator) with examples including U.S. Pat. No. 4,533,082 and EP 0431992. This enables the delivery of relatively well monodispersed droplets without requiring the pressure waves to be transmitted through a liquid layer further increasing efficiency and enabling a wider range of embodiments. A preferred embodiment of such a device such as described in U.S. Pat. No. 5,518,179 uses a bending mode actuator to deliver the vibrational energy to the membrane as this enables the use of thin low cost actuators and further increases efficiency.
Often it is desirable to use a master-cartridge model in which a master unit can spray liquid contained in a replaceable cartridge. Preferably, all liquid contacting components reside on the cartridge and as many non-liquid contacting components as possible reside on the master. This minimises the cost of the cartridge whilst avoiding liquid cross-contamination between cartridges and liquid contamination of the master. Examples of fields where such an approach finds use are the medical field and the consumer fragrance field. In the medical field dose sterility can be critical and this can be achieved by containing each dose in its own cartridge (or capsule). Also in the medical field the same master device may be designed to be used with more than one patient and cross-contamination should be avoided. In the fragrance field, each cartridge may contain a different fragrance and again cross-contamination should be avoided. Other fields in which similar requirements are met will be obvious to someone skilled in the art.
One approach to avoid cross contamination is to place the perforate membrane and actuator into the cartridge component with the electronics and power source in the master. This limits the required connection between the two components to electrical but, with the actuator in the cartridge, leaves a relatively high cost component in the cartridge. Further, and more importantly for medical applications where each cartridge contains a single dose, the cartridge size may be relatively large compared to the amount of liquid it contains. There is therefore a need to move the actuator out of the cartridge component leaving just the liquid contacting perforate membrane as this approach can reduce both cartridge cost and size.
The requirement to avoid cross contamination is known in the art and, for relatively inefficient applications where low power consumption is not crucial, solutions have been proposed. U.S. Pat. No. 3,561,444 teaches, for a pond-mister style device, using a liquid that is not dispensed to provide the connection between the vibration element in the master and the surface to be vibrated in the cartridge. U.S. Pat. No. 4,702,418, WO 2006/006963, WO 2009/150619, WO 2010/026532 and WO 2009/136304 teach various means of connecting the vibration force to a surface in the cartridge that is situated in close proximity to a perforate membrane with the vibration then transmitted through the liquid to be sprayed. EP 1,475,108 and U.S. Pat. No. 5,838,350 teach coupling of a piezoceramic component directly to a perforate membrane but do not teach how this can be done in an efficient manner or without the connection approach resulting in excessive energy absorption. The Büchi B-90 Nano Spray Drier enables the perforate membrane to be replaced by requiring the user to screw the membrane onto the actuator using a custom nut to a specified torque level. Whilst this is suitable for a laboratory instrument the replacement process is hard to automate in a compact device it would not be acceptable for a device that is designed to be operated by a consumer for example.
Efficient connection of energy is even more critical for low power devices and in particular for devices where the actuator operates in bending mode as in U.S. Pat. No. 5,518,179. Further, efficient connection of energy through a bending interface is significantly more challenging than efficient connection of energy through a translating interface. This is because a torque in addition to a normal force must be transmitted and also because any structures that result in the device becoming thicker (a screw thread for example) reduce vibration.
In summary, there is a requirement for a means to enable vibration to be effectively transmitted from an actuator to a perforate membrane in which the perforate membrane can be easily removed and replaced by a non-skilled consumer or automatically within a compact device. Such transmission would ideally not absorb excessive vibration energy. Such transmission would ideally not reduce the vibration amplitude of the perforate membrane. These preferable requirements are especially challenging with bending-mode actuator devices as they are more easily damped.